It is common practice to detect angular positions of a rotor of a brushless electric motor by means of a magnetoelectric transducer in order to control the currents flowing into the stator coil of the brushless motor. Record players are known which have a plurality of magnets arranged on the periphery of a turntable for speed control. To this end, a magnetic head has been located adjacent to the periphery of the turntable. An alternating magnetic field occurs at the magnetic head with rotation of the turntable. The output signal for one whose frequency is proportional to the rotational speed of the turntable, and this signal has then been applied to a servo-amplifier to control the driving motor of the turntable.
Such a prior art system has the disadvantage that the output signal from the magnetic head is greatly changed with the gap between the magnetic head and the turn table. It is also apt to be affected by a change of temperature and also by an external magnetic field. These disadvantages have been overcome by use of a magnetoresistive element.